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Black Twig Borers

Community Contributed

By Glenn I. Teves, County Extension Agent, UH CTAHR

One of the obvious long-term impacts of drought on plants and trees is stress that can lead to death of branches or even the entire tree. Older trees are especially susceptible since they’re weaker due to age, and they lack vigor or juvenility. Insects will zero in on them and attack stems and eventually heartwood. Older wood is harder and dryer, and are especially attractive to insects, especially beetles. When you see symptoms such as dead branches, the damage had already occurred months earlier. One cue of this problem on Molokai is dying branches of Eucalyptus trees in the mountains. Along the mauka edge of Ironwood Hills Golf Course, branches of Eucalyptus have been dying back for some time now from insect damage.

There are many culprits sensing that plants are under stress and attack. Of these, the Black Twig Borer, also known as the Coffee Twig Borer, is notorious for its ability to kill large trees. A black beetle about 1/8-inch long, it can bring giant trees down and even attack large healthy trees, but stressed plants are more susceptible. Belonging to a group known as Ambrosia beetles, they bore into dead and weakened trees and release spores of a fungus on which they feed on. This same fungus will clog the plants water intake system, killing branches and whole trees.

First found in Hawaii in 1961, the Black Twig Borer is native to Asia and has spread to all coffee growing areas of the world. The black twig borer has a very wide host range and will attack over 200 species of plants and trees including orchids, anthuriums, citrus, coffee, cacao, paper bark, lychee, macadamia, mango, koa haole, Christmas berry, guava, kukui, hibiscus, pikake, mahogany, Surinam cherry, and numerous Eucalyptus species, among others. We have been receiving reports of dying trees, and this would be the most obvious culprit.

Female black twig borers will tunnel into woody twigs, leaving pin-sized entry holes. Once inside, they will excavate galleries and lay eggs. It is here where they also introduce a fungus, including the Fusurium fungus, known for clogging the plants water intake system leading to decline and death. Males will stay in the galleries, while females will breed and exit the pin holes, establishing a new gallery elsewhere, including in an adjacent tree.

The key to disease and insect control is to grow a healthy plant. Maintaining trees by adequately watering and feeding, and also pruning dead and diseased material, is essential to keeping plants and trees healthy.